Operation Manual – Routing Protocol
H3C S5600 Series Ethernet Switches
Chapter 4 OSPF Configuration
4-8
The virtual link between the two ABRs acts as a point-to-point connection. Therefore,
you can configure interface parameters such as hello packet interval on the virtual link
as they are configured on physical interfaces.
The two ABRs on the virtual link exchange OSPF packets with each other directly, the
OSPF routers in between simply convey these OSPF packets as normal IP packets.
IV. (Totally) Stub area
The ABR in a stub area does not distribute Type-5 LSAs into the area, so the routing
table scale and amount of routing information in this area are reduced significantly.
You can also configure the stub area as a Totally Stub area, where the ABR advertises
neither the routes of other areas nor the external routes.
Stub area configuration is optional, and not every area is qualified to be a stub area. In
general, a stub area resides on the border of the AS.
The ABR in a stub area generates a default route into the area.
Note the following when configuring a (totally) stub area:
z
The backbone area cannot be a (totally) stub area
z
The
stub
command must be configured on routers in a (totally) stub area
z
A (totally) stub area cannot have an ASBR because AS external routes cannot
be distributed into the stub area.
z
Virtual links cannot transit (totally) stub areas.
V. NSSA area
Similar to a stub area, an NSSA area imports no AS external LSA (Type-5 LSA) but
can import Type-7 LSAs that are generated by the ASBR and distributed throughout
the NSSA area. When reaching the NSSA ABR, Type-7 LSAs are translated into
Type-5 LSAs by the ABR (the Type-7 LSAs translator state must be
enabled/elected
)
before being advertised to other areas.
In the following figure, the OSPF AS contains three areas: Area 1, Area 2 and Area 0.
The other two ASs employ the RIP protocol. Area 1 is an NSSA area, and the ASBR in
it translates RIP routes into Type-7 LSAs and advertises them throughout Area 1.
When these LSAs reach the NSSA ABR with the Type-7 LSAs translator state as
Enabled
or
Elected
, the ABR translates Type-7 LSAs to Type-5 LSAs for
advertisement to Area 0 and Area 2.
On the left of the figure, RIP routes are translated into Type-5 LSAs by the ASBR of
Area 2 and distributed into the OSPF AS. However, Area 1 is an NSSA area, so these
Type-5 LSAs cannot travel to Area 1.
Similar to stub areas, virtual links cannot transit NSSA areas.
